5.8 Handling Local Macros

Feature tests offered by Autoconf do not cover all needs. People often have to supplement existing tests with their own macros, or with third-party macros.

There are two ways to organize custom macros in a package.

The first possibility (the historical practice) is to list all your macros in acinclude.m4. This file will be included in aclocal.m4 when you run aclocal, and its macro(s) will henceforth be visible to autoconf. However if it contains numerous macros, it will rapidly become difficult to maintain, and it will be almost impossible to share macros between packages.

The second possibility, which we do recommend, is to write each macro in its own file and gather all these files in a directory. This directory is usually called m4/. To build aclocal.m4, one should therefore instruct aclocal to scan m4/. From the command line, this is done with aclocal -I m4. The top-level Makefile.am should also be updated to define

 ACLOCAL_AMFLAGS = -I m4

ACLOCAL_AMFLAGS contains options to pass to aclocal when aclocal.m4 is to be rebuilt by make. This line is also used by autoreconf (see Using autoreconf to Update configure Scripts in The Autoconf Manual) to run aclocal with suitable options, or by autopoint (see Invoking the autopoint Program in GNU gettext tools) and gettextize (see Invoking the gettextize Program in GNU gettext tools) to locate the place where Gettext’s macros should be installed. So even if you do not really care about the rebuild rules, you should define ACLOCAL_AMFLAGS.

When aclocal -I m4 is run, it will build a aclocal.m4 that m4_includes any file from m4/ that defines a required macro. Macros not found locally will still be searched in system-wide directories, as explained in Macro search path.

Custom macros should be distributed for the same reason that configure.ac is: so that other people have all the sources of your package if they want to work on it. Actually, this distribution happens automatically because all m4_included files are distributed.

However there is no consensus on the distribution of third-party macros that your package may use. Many libraries install their own macro in the system-wide aclocal directory (see Writing your own aclocal macros). For instance Guile ships with a file called guile.m4 that contains the macro GUILE_FLAGS which can be used to define setup compiler and linker flags appropriate for using Guile. Using GUILE_FLAGS in configure.ac will cause aclocal to copy guile.m4 into aclocal.m4, but as guile.m4 is not part of the project, it will not be distributed. Technically, that means a user which needs to rebuild aclocal.m4 will have to install Guile first. This is probably OK, if Guile already is a requirement to build the package. However, if Guile is only an optional feature, or if your package might run on architectures where Guile cannot be installed, this requirement will hinder development. An easy solution is to copy such third-party macros in your local m4/ directory so they get distributed.